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How does stator diameter/height affects motor torque/power
#1
We have just seen DYS's new SE2008 2550KV motor, and that makes me wonder whether there is higher gain in power/torque from increasing motor stator height, or diameter. This is especially important as every gram counts when it comes to racing mini quads. 

What do you think?
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#2
Great question, I've always wanted to understand what the impact of these are!
QAV210 - LUX FC - LB 20A ESC - Cobra 2205 2300
QAV210 - LUX FC - DYS XS20A ESC - DYS SE2205 2300
Alien 5" - KISS FC - KISS 24A ESC - Lumenier 2206 2350

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#3
I have always thought that the larger diameter motors had more torque, while the taller motors had faster acceleration.

I am not sure if i am right, but the mechanics seem right. Larger diameter motors have the tangential pulling and pushing forces further from the axis of rotation, and therefore have a longer lever arm to generate the rotational torque (torque=r*F, yes there's a sin theta in there but if the force is tangential then that is 1). The downside is that the mass of the motor bell is further from the axis of rotation, so the larger diameter motor has more rotational inertia and therefore is slower to respond to changes in rotation.

A smaller diameter motor has less rotational inertia and is therefore better able to react to attitude changes, but is also weaker. Then if you lengthen that smaller diameter motor you get more torque, albeit not as much as you get from a enlarging the diameter. I would have to look into the equations to be sire, but my gut tells me that torque increases in a linear relationship with stator length, but increases with the square of the radius. I might not be exactly right about the ratio, but in general torque is more sensitive to radius than to length.

I think the reason these 2008 motors surprised you with their weight is because mass is more sensitive to length than radius. When you increase the radius, you essentially have to add a little strip of mass to fill in the perimeter where the circumference has expanded. When you increase length, you have to add a full ring of mass at whatever the diameter is.

Sorry if this message was either too complicated or too simple - I didn't take the time to actually work out any of the math, so this is all what we'd call "back of the napkin" analysis Smile
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#4
I thought you explained it very well.
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#5
(06-Aug-2016, 11:57 PM)zafuquazar Wrote: I have always thought that the larger diameter motors had more torque, while the taller motors had faster acceleration.

This make sense to me, but it makes me ask:  Which is more important for fpv racing?  More torque or faster acceleration? (And why?)  Huh
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#6
I vote put it on a quad and fly it like you stole it and see if there's a difference. just saying lol Big Grin
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#7
(07-Aug-2016, 01:55 AM)sloscotty Wrote: This make sense to me, but it makes me ask:  Which is more important for fpv racing?  More torque or faster acceleration? (And why?)  Huh

Yes, that's the question. Generally, i would think that the bigger the prop or higher the pitch, the more torque you would want. That's a no brainier because those torquey props will not react quickly anyway, so there's no downside to more torque. So aggressive 5" props (tri blade BN) and 6" props would want 2205s. Then on the other end of the spectrum, like 3" or smaller, you'd want fast rotational acceleration, which goes along with higher kv motors. 

But it also depends on what you are wanting to do with the copter. Clearly a slow moving heavy lifter or camera craft would be all about torque.  Maybe racing wants high torque and acro wants fast acceleration?  For FPV racing, I think you want as much speed as you can get (aggressive props and high torque motors). Yes, you want quick acceleration, but if you can hold smooth lines, you aren't really asking the copter to do that much acceleration in any direction - at least not compared to acro...

For everyday quads that we use for flying around having fun with a mix of high speed and acro (I say it as if my piloting skills were up to the task), probably the best motor would be one that was just torquey enough to spin the props we want to use and therefore quicker to react than a motor that had more torque than we need.

I had a conversation about a similar topic with EngineerX recently. We were discussing the best motor for a 4" quad and I went into that conversation thinking that a high kv 1806 was a better choice than what has become the more popular choice of 2205 2300. I was thinking that 2205 2300 was too slow and needlessly heavy for a 4" quad. EngineerX has tested hundreds of motor ad prop combos now and his advice was to indeed go with 2204 or 2205 2300kv for 4". He agreed that the 1806 2950kv motors that I wanted to use would be good for a bi-blade non bullnose prop. The problem is that I don't really want to run those props. I want to run DAL or other durable prop so I don't have to keep replacing them, and they are generally more aggressive and physically heavier. The high kV 1806 cannot handle those props. I ended up buying the ZMX V2 2204 2300kv motors for that quad (can't report on the result yet - still building). I've heard it from many guys before and I agree - pick your props first and pick your motors based on the props...
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#8
Makes perfect sense Zafuquazar!
I don't know much about making motors Smile I guess there is also consideration motor weight, obviously when there is an increase in motor width or height, it comes with additional weight.
Manufacturers and testers must take into account the gain in performance per gram of additional weight, to decide what is more suitable for the motors they plan to build. (gain in performance includes torque, power, acceleration etc)
It can get fairly complicated I think...
Don't be a LOS'er, be an FPV'er :)  My Gear - Facebook - Instagram - Twitter
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#9
(06-Aug-2016, 07:26 PM)Sigloss Wrote: but my gut tells me that torque increases in a linear relationship with stator length, but increases with the square of the radius

Torque is somewhat a function of angular acceleration ε and moment of inertia I, while the moment of inertia I for a solid cylinder (let's see motor as a solid cylinder) is proportional to r^2, so it seems you are correct. Moment of inertia is also proportional to mass, and mass scales mostly linearly with cylinder's length.

[Image: ql_46f326f8e18b34f7dd21ec0f04272621_l3.png]
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#10
Must have an editing glitch there, that statement was far too intelligent to be attributed to me Smile
QAV210 - LUX FC - LB 20A ESC - Cobra 2205 2300
QAV210 - LUX FC - DYS XS20A ESC - DYS SE2205 2300
Alien 5" - KISS FC - KISS 24A ESC - Lumenier 2206 2350

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#11
(11-Aug-2016, 01:53 PM)Sigloss Wrote: Must have an editing glitch there, that statement was far too intelligent to be attributed to me Smile

Seems like auto-quotation by selecting a region has issues :-)
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#12
(10-Aug-2016, 01:00 PM)Andrey Mironov Wrote: Torque is somewhat a function of angular acceleration ε and moment of inertia I, while the moment of inertia I for a solid cylinder (let's see motor as a solid cylinder) is proportional to r^2, so it seems you are correct. Moment of inertia is also proportional to mass, and mass scales mostly linearly with cylinder's length.

[Image: ql_46f326f8e18b34f7dd21ec0f04272621_l3.png]

sometimes I get lucky Smile
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